Manufacturing method of semiconductor structure

ABSTRACT

A method for forming a semiconductor structure includes: providing a semiconductor substrate having a first pad and a second pad on a top surface of the semiconductor substrate; providing a circuit board having an active pad and a non-metallic surface; providing a first solder ball and a second solder ball on the active pad and the non-metallic surface respectively; attaching the first pad and the second pad on the first solder ball and the second solder ball respectively; and reflowing the first solder ball and the second solder ball to form a first bump wetted on the active pad and a second bump not wetted on the non-metallic surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/152,274, filed on May 11, 2016, which is a continuation of U.S.application Ser. No. 14/063,302, filed on Oct. 25, 2013, which isincorporated by reference in its entirety.

FIELD

The disclosure relates to a structure, and more particularly to asemiconductor structure and a manufacturing method of the semiconductor.

BACKGROUND

Chip scale packages (CSP) are widely adopted for semiconductor chipassemblies in the industry because the component has a smaller size. Apopular methodology of manufacturing a CSP component is a technologycalled surface mounting technology (SMT). The surface mountingtechnology is a method in which the semiconductor chip is mounted orplaced directly on the surface of a printed circuit board (PCB). Asemiconductor component made with SMT usually has either smaller bondingwires or no bonding wires at all.

The semiconductor component made with SMT is smaller in size and lowerin electrical resistance. During manufacturing of the SMT semiconductorcomponent, a ball grid array (BGA) is a type of packaging used forintegrating the semiconductor chip and the printed circuit board. In BGApackaging, a number of solder balls are first disposed on either thepads of the semiconductor chip or the pads of the printed circuit board,and then the semiconductor chip and the circuit board are bonded throughthe solder balls.

By replacing the wires with the solder balls in the surface mountingcomponent, the resistance and required package space are both reduced.However, similar to the bonding wires of the other packaging process,the reliability of the solder balls affects the manufacturing yield ofthe surface mounting technology. As such, improvements in the integrityand method for a solder ball to form in a surface mounting componentcontinue to be sought.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a cross sectional view of a semiconductor structure having abump contacting an electrically insulative top surface on a circuitboard in accordance with some embodiments of the present disclosure.

FIG. 1A is a cross sectional view of a semiconductor structure having abump contacting an electrically insulative top surface on a circuitboard in accordance with some embodiments of the present disclosure.

FIG. 2 is a cross sectional view of a semiconductor structure having abump contacting with an insulative pad on circuit board in accordancewith some embodiments of the present disclosure.

FIG. 2A is a cross sectional view of a semiconductor structure having abump contacting with an insulative pad in a recessed portion of acircuit board in accordance with some embodiments of the presentdisclosure.

FIG. 3 is a cross sectional view of a semiconductor structure having abump contacting with an insulative pad on circuit board in accordancewith some embodiments of the present disclosure.

FIG. 4 is a flowchart of a method 400 for fabricating a semiconductorstructure bump according to various aspects of the present disclosure.

FIGS. 5A to 5E are perspective views of a method of forming asemiconductor structure in accordance some embodiments of the presentdisclosure.

FIG. 6 is the cross sectional view along line AA′ in FIG. 4E inaccordance some embodiments of the present disclosure.

FIG. 7 is the cross sectional view along line AA′ in FIG. 4E inaccordance some embodiments of the present disclosure.

FIGS. 8A-8C are top views of a pad array on a circuit board inaccordance some embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The manufacturing and use of the embodiments are discussed in detailsbelow. It should be appreciated, however, that the embodiments providemany applicable inventive concepts that can be embodied in a widevariety of specific contexts. It is to be understood that the followingdisclosure provides many different embodiments or examples forimplementing different features of various embodiments. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting.

Further, it is understood that several processing steps and/or featuresof a device may be only briefly described. Also, additional processingsteps and/or features can be added, and certain of the followingprocessing steps and/or features can be removed or changed while stillimplementing the claims. Thus, the following description should beunderstood to represent examples only, and are not intended to suggestthat one or more steps or features is required.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

“Bump” is used as the connection member between the semiconductor deviceand the circuit board after the bonding processes. The different typesof bumps are formed using a soldering material, for example, solderballs, or solder pastes.

For a surface mounting component or a chip scale package assembly,solder balls are bonded on the pads of a semiconductor device or acircuit board and form bumps to connect the semiconductor device withthe circuit board. Stress distribution inside the bump is an issue. Highstress in the corners of the bumps or on the contact surface between thebumps and the pad bonded thereon is causing the bumps to be vulnerableand may cause cracking and a bad electrical connection.

In the present disclosure, a semiconductor structure or a SMT componenthaving a number of bumps are bonded on a semiconductor device andconfigured to connect the semiconductor device with a circuit board. Thebumps are elongated to have a smaller wetting angle with the pads of thesemiconductor device or circuit. Each of the elongated bumps is designedto be capable of having a low stress distributed at the corners of thebumps.

A method according to the present disclosure is designed to at least twodifferent types of bumps in a surface mounting component or a chip scalepackage assembly. The first type is called active bump because the firsttype bump is conducting electrical current between the circuit board andthe semiconductor device. The second type is non-active bump because thesecond type bump is not electrically connected with any circuitryneither in the circuit board nor the semiconductor device. The secondtype bump acts as a dummy bump.

In the present disclosure, connection between non-active bump and thecircuit board is in non-wetted manner. Active bump is elongated by anon-wetted non-active bump. Active bump is elongated to have a greateraspect ratio than the non-active bump. In some embodiments, an activebump is taller than an adjacent non-active bump.

FIG. 1 is a semiconductor structure 100. In some embodiments, thesemiconductor structure 100 is a surface mounting component including asemiconductor device 105 and a circuit board 109. The semiconductordevice 105 has a semiconductor substrate 102. A “semiconductorsubstrate” refers to a bulk semiconductor substrate on which variouslayers and device structure are formed. In some embodiments, the bulksubstrate includes silicon or a compound semiconductor, such as Ga As,InP, Si/Ge, or SiC. Examples of the layers include dielectric layers,doped layers, polysilicon layers or conductive layers. Examples of thedevice structures include transistors, resistors, and/or capacitors,which may be interconnected through an interconnect layer to additionalintegrated circuits. Pads 104 are disposed on a top surface 102-1 of thesemiconductor substrate 102. Pads 104 are electrically connected to aninternal circuit in the semiconductor substrate 102 at one side andelectrically connected to an active pad 108 of the circuit board 109 viaa bump 120. The pad 108 is disposed on a top surface 109-1 of thecircuit board 109. In some embodiments, the top surface 109-1 is not aflat surface as in FIG. 1. The top surface 109 includes some recessedportions 109-1 a to accommodate pads 108. The circuit board 109 isdesigned to mechanically supports and electrically connects electroniccomponents using conductive tracks, pads and other features etched fromconductor sheets such as copper, laminated onto a non-conductivesubstrate. In some embodiments, the circuit board 109 is a printedcircuit board (PCB). A PCB populated with electronic components iscalled a printed circuit assembly (PCA), printed circuit board assemblyor PCB assembly (PCBA).

Pads 104 are active pads. Active pad herein means the pad is conductiveand located on an electric current path connecting the circuit board 109and the semiconductor substrate 102. Like in FIG. 1, an active pad 104on the semiconductor substrate 102 is connecting with a bump 120 whereasan active pad 108 on the circuit board 109 is connected with. Thus,electrical current travels from either active pad 104 or 108 to theother side of the bump 120.

On the contrary, non-active pad means that the pad is not on an electriccurrent path. In some embodiments, it is also called a dummy pad.Referring to FIG. 1, a non-active pad 104′ is disposed on the topsurface 102-1 but electrically insulated from the internal circuit ofthe semiconductor substrate 102. The non-active pad 104′ is locatedcorrespondingly to a contact area 108′ on the top surface 109-1 of thecircuit board 109. In semiconductor structure 100, the non-active pad104′ is connected to a non-active bump 120′. At the other end of thebump 120′, the bump 120′ is in contact with an electrically insulativecontact area 108′, therefore there is no electric current passingthrough the non-active pad 104′ while the semiconductor structure 100 isin operation. In some embodiments, the contact area 108′ is anon-metallic surface. In some embodiments, the non-active pad 104′ is ametallic layer such that the bump 120′ is in contact with the non-activepad 104′ in wetted manner.

In some embodiments, the surface of electrically insulative contact area108′ is configured to be unwettable to the bump. In some embodiments,the surface of electrically insulative contact area 108′ is a portion ofan electrically insulative coating of circuit board 109. The surface iscoated with an insulative material that can not provide an adhesiveforce to overcome cohesive force of the bump when reflowed. The bumpmaintains a curvature surface at the point contacting with theelectrically insulative contact area 108′. Therefore, bump such as 120′is in contact with the contact area 108′ in non-wetted manner. There isno any metallic bonding between bump 120′ and the contact area 108′. Insome embodiments, the insulative material includes Acrylic, Epoxy,Polyurethane, Polyurethane, Silicones. The insulative material is easeof rework and operation in simple drying process, good moistureresistance, chemical resistant, and high dielectric constant.

Pads 108 on the circuit board 109 are designed to include conductivematerial such as copper, aluminum, gold, silver, or alloys thereof,therefore the top surface of pads 108 provide an adhesive force adequateenough to overcome cohesive force of the bump when reflowed. Thus, theactive bump 120 is in contact with the pad 108 in wetted manner. Theactive bump 120 has a greater contact area surface with pad 108 than thenon-active bump 120′ with contact area 108′. When a substantially sameamount of solder material is applied to form bump 120 and 120′, theactive bump 120 has a higher aspect ratio than the non-active bump 120′.

Referring to FIG. 1A, the aspect ratio of each bump is defined as theratio of the is height of the bump divided by the central width (fullwidth at half height). In some embodiments, solder material between pad104 and pad 108 is elongated by solder material between pads 104′ and108′ during reflow. The non-wetted curvature surface of the soldermaterial on pad 108′ increase the gap between circuit board 109 andsemiconductor substrate 102. Thus, wetted soldering material iselongated to have a higher aspect ratio and a smaller wetting angle withpad 108. The elongated solder material or bump are more resistant to anycrack induced by stress. In FIG. 1A, bump 120 has a height H₁ and acentral width W₁, and bump 120′ has a height H₂ and a central width W₂.The bump 120 has an aspect ratio H₁/W₁ and the bump 120′ has an aspectratio H₂/W₂. In some embodiments, H₁/W₁ is greater than H₂/W₂.

The active bump 120 has a wetting angle, Φ, which is the contact anglebetween the bump 120 and the active pad 108 of the circuit board 109. Insome embodiments according to the present disclosure, the wetting angleΦ is substantially smaller than or equal to 90 degrees. In someembodiments, the wetting angle Φ is substantially between about 80degrees and 110 degrees. In some embodiments, the wetting angle Φ issubstantially between about 80 degrees and 130 degrees.

According to some embodiments of the present disclosure, the contactarea 108′ in contact with the bump 120′ is an electrically insulativepad. As in FIG. 2, a pad 107 is disposed on surface 109-1. The surface109-1 includes several substantially identical recessed portions 109-1 aand each recessed portion is configured to accommodate either an activepad 108 or a non-active pad 107. In some embodiments, the non-active pad107 has a same thickness with the active pad 108. However, top surfaceof the non-active pad 107 includes material not suitable for solderwetting. In some embodiments, the non-active pad 107 includes polymericmaterial. In some embodiments, the non-active pad 107 includes materialsuch as synthetic rubber, phenol formaldehyde resin (or Bakelite),neoprene, nylon, polyvinyl chloride (PVC or vinyl), polystyrene,polyethylene, polypropylene, polyacrylonitrile, PVB, silicone. The pad107 is located correspondingly to a non-active pad 104′ on thesemiconductor substrate 102. Because bump 120′ does can not be wetted onthe top surface of the non-active pad during reflow, the bump 120′maintains a curvature tip at one end. Bump wetted on pad 108, such as120, is elongated to have a smaller width than the non-wetted bump 120′.Because both wetted bump 120 and non-wetted bump 120′ have a sameheight, the aspect ratio of wetted bump 120 is greater than thenon-wetted bump 120′.

In some embodiments as in FIG. 2A, the pad 107 on circuit board 109 isthicker than the pad 108. The pad 107 has an extrusion d over the topsurface 109-1 of circuit board 109. The extrusion d is betweenpredetermined ranges. In some embodiments, extrusion d is about onetenth of the thickness of the pad 107. In some embodiments, extension dis between about one eighth and one tenth of the thickness of the pad107.

In some embodiments as illustrated in FIG. 3, the pad 107 is disposed ona portion of the top surface 109-1 that is at a higher level than therecessed portions 109-1 a. In other words, the pad 107 is disposed at asurface that is higher than the active pad 108. Active bumps 120 are incontact with pad 108 in wetted manner, however, the non-active bump 120′is contacting with pad 107 in non-wetted manner. Because the spacebetween semiconductor substrate 102 and circuit board 109 is constrainedat a fixed value S, the wetted bump 120 is elongated by non-wetted bumps120′ to have a taller thickness than the non-wetted bump 120′. Moreover,the wetted bump 120 also has a smaller wetting angle Φ₁ than the wettingangle Φ₂ of the non-wetted bump 120′. In some embodiments, Φ₁ is about10% to 15% smaller than Φ₂. In some embodiments, Φ₁ is about 10% to 30%smaller than Φ₂.

FIG. 4 is a flowchart of a method 400 for fabricating a semiconductorstructure with non-wetted bump according to various aspects of thepresent disclosure. The method 400 includes operation 402 in which asemiconductor substrate is provided. The method 400 continues withoperation 404 in which a pad is formed on a top surface of thesemiconductor substrate. The method 400 continues with operation 406 inwhich a circuit board is provided. On the circuit, a contact areacorresponding to the pad on the semiconductor substrate is provides. Themethod 400 continues with operation 408 in which a bump is formedbetween the semiconductor substrate and the circuit. The bump contactsthe contact area provided on the circuit board in non-wetted manner. Thevarious operations of FIG. 4 are discussed below in more detail inassociation with cross sectional or top views corresponding to theoperations of the flow diagram.

In FIG. 5A, a semiconductor substrate 102 is provided. Several pads(104, 104′) are formed on a top surface 102-1 of the semiconductorsubstrate 102. The pads are arranged in an array. In some embodiments,pads are designed for a SMT ball grid array (BGA) package. Some padslike 104 are active pads and some pads like 104′ are non-active or dummypads. For a better identification, active pads 104 and non-active pads104′ are drawn in different patterns. However, the pattern differenceshould not be deemed as a restriction to the present disclosure.

In FIG. 5B, a circuit board 109 is provided. On a top surface 109-1 ofthe circuit board 109, there are several non-metallic contact areas 108′are designed correspondingly to the non-active pad 104′ in FIG. 5A. Thecircuit board 109 also has some recessed portions 109-1 a, which aredesigned to accommodate active pads 108. The distribution pattern of theactive pads 108 on circuit board 109 is corresponding to thedistribution pattern of the active pads 104 on the semiconductorsubstrate 102 in FIG. 5A. In some embodiments, the contact area 108′ isa part of the top surface 109-1 hence that is not distinguishable fromthe top surface 109-1. Dotted line is used to mean that the boundarybetween the contact area 108′ and the top surface 109-1 may be invisiblefor some embodiments.

According to some embodiments, solder balls 122 are placed on pads 104and 104′ as in FIG. 5C. The placement is performed in various manners,such as ball drop, stencil. Some operations such as flux pasting areskipped herein. The solder balls 122 are reflowed in order to be bondedon the top surface of the pads 104 and 104′. In FIG. 5D, thesemiconductor substrate 102 is flipped to have its top surface 102-1facing the top surface 109-1 of circuit board 109. Each solder ball 122bonded with a pad 104 or 104′ is contacting a corresponding location ofcircuit 109. For example, a solder ball 122 bonded on an active pad 104(not visible) is in contact with an active pad 108; a solder ball bondedon a non-active pad 104′ is in contact with a contact area 108′.

A reflow is performed to have solder balls 122 bonded with the activepads 108. In FIG. 5E, solder balls are heated to become bumps 120 and120′. Solder balls contacting with active pads 108 are bonded in wettedmanner, however, solder balls contacting with contact areas 108′ are notbonded since the solder material can not be wetted on the surface of thecontact area 108′. FIG. 6 is a cross sectional view along line AA′ inFIG. 5E. There are two wetted bumps 120 between two non-wetted bumps120′.

In some embodiments, the contact areas 108′ are replaced with apolymeric pad. As in FIG. 7, some polymeric pads 107 are placed on thetop surface 109-1 to be in contact with solder balls. The polymeric pads107 are formed in various manners. In some embodiments, a polymer layeris coated on the top surface 109-1 and a photolithography operation isperformed to from pads 107. In some embodiments, a polymeric material iscoated on the top surface 109-1. A photo mask is further placed on thepolymeric material followed by an etching. The polymeric is patterned toform the pads 107. In some embodiments, polymer is preformed intoseveral disks and the preformed polymeric disks are placed on the topsurface 109-1 in order to form the polymeric pads 107. In someembodiments, polymeric pads 107 are placed in recessed portions of thetop surface 109-1. An adhesive may be used to glue the pads 107 on topsurface 109-1.

The non-wetted members such as contact area 108′ or pads 107 arearranged in various patterns. FIG. 8A to 8C are top views of a circuitboard 109 in accordance with some embodiments of present disclosure. Onthe top surface 109-1, a pad array is formed. In some embodiments, thenon-wetted members are arranged symmetrical to a center of the array. Asin FIG. 8A, some polymeric pads 107 are arranged at corners of thearray. Point O is a visual center point of the array. The non-wettedbumps are formed on the polymeric 107 symmetrically to the center pointO. A symmetrical arrangement can help form a more uniform elongated bumpin the array. In some embodiments, some polymeric pads 107 are arrangedsymmetrically at edges and corners of the array as in FIG. 8B. In someembodiments, polymeric pads 107 are arranged inside the array as in FIG.8C. In some embodiments, the pads 107 are replaced with contact areas108′ (not illustrated), which are also arranged symmetrical to thecenter O.

In some embodiments, a method for forming a semiconductor structurecomprising: providing a semiconductor substrate having a first pad and asecond pad on a top surface of the semiconductor substrate; providing acircuit board having an active pad and a non-metallic surface; providinga first solder ball and a second solder ball on the active pad and thenon-metallic surface respectively; attaching the first pad and thesecond pad on the first solder ball and the second solder ballrespectively; and reflowing the first solder ball and the second solderball to form a first bump wetted on the active pad and a second bump notwetted on the non-metallic surface.

In some embodiments, a method for forming a semiconductor structurecomprising: providing a semiconductor substrate having a first pad and asecond pad on a top surface of the semiconductor substrate; providing acircuit board having an active pad and a non-metallic surface; providinga first solder ball and a second solder ball on the active pad and thenon-metallic surface respectively; attaching the first pad and thesecond pad on the first solder ball and the second solder ballrespectively; and reflowing the first solder ball and the second solderball to form a first bump and a second bump respectively; wherein thefirst bump is arranged to have a first contact angle formed between afirst contact surface of the first bump on the active pad and a surfaceof the active pad, the second bump is arranged to have a second contactangle formed between a second contact surface of the second bump on thenon-metallic surface and a surface of the non-metallic surface, and thefirst contact angle is different from the second contact angle.

In some embodiments, a method for forming a semiconductor structurecomprising: providing a semiconductor substrate, having a plurality offirst pads and a plurality of second pads on a top surface of thesemiconductor substrate; providing a circuit board having a plurality ofactive pads and a plurality of non-metallic surfaces; providing aplurality of first solder balls respectively on the plurality of activepads, and a plurality of second solder balls respectively on theplurality of non-metallic surfaces; attaching the plurality of firstpads and the plurality of second pads on the plurality of first solderballs and the plurality of second solder balls respectively; andreflowing the plurality of first solder balls and the plurality ofsecond solder balls to form a plurality of first bumps and a pluralityof second bumps respectively; wherein the plurality of active pads andthe plurality of non-metallic surfaces are formed as an array on thecircuit board, and the plurality of non-metallic surfaces are arrangedto be symmetrical to a center of the array.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As those skilled in the art will readilyappreciate form the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure.

Accordingly, the appended claims are intended to include within theirscope such as processes, machines, manufacture, and compositions ofmatter, means, methods or steps. In addition, each claim constitutes aseparate embodiment, and the combination of various claims andembodiments are within the scope of the invention.

What is claimed is:
 1. A method for forming a semiconductor structure,comprising: providing a semiconductor substrate, having a first activepad and a dummy pad on a first top surface of the semiconductorsubstrate; providing a circuit board having a second active pad and anon-metallic surface, wherein a second top surface of the second activepad is at substantially same level as the non-metallic surface;providing a first solder ball and a second solder ball on the second topsurface and the non-metallic surface respectively; attaching the firstactive pad and the dummy pad on the first solder ball and the secondsolder ball respectively; and reflowing the first solder ball and thesecond solder ball to form a first bump wetted on the second top surfaceand a second bump not wetted on the non-metallic surface, and toelongate the first bump, wherein the first active pad and the first bumpare electrically connected to an internal circuit of the semiconductorsubstrate, and the dummy pad and the second bump are electricallyinsulated from the internal circuit.
 2. The method of claim 1, whereinthe first bump is substantially taller than the second bump.
 3. Themethod of claim 1, further comprising: providing a recessed portionrecessed into the circuit board for accommodating the second active pad.4. The method of claim 1, wherein a first contact angle is formedbetween a first contact surface of the first bump on the second activepad and the second top surface of the second active pad, a secondcontact angle is formed between a second contact surface of the secondbump on the non-metallic surface and a portion of the non-metallicsurface, and the second angle is different from the first angle.
 5. Themethod of claim 4, wherein the first contact angle and the secondcontact angle are greater than 90 degrees, and the first contact angleis smaller than the second contact angle.
 6. The method of claim 1,wherein a first height of the first solder ball is equal to a secondheight of the second solder ball, and a maximum width of the first bumpis different from a maximum width of the second bump.
 7. The method ofclaim 6, wherein the maximum width of the second bump is greater thanthe maximum width of the first bump.
 8. The method of claim 1, whereinthe first solder ball has a first amount of solder material, the secondsolder ball has a second amount of solder material, and the first amountof solder material is equal to the second amount of solder material. 9.A method for forming a semiconductor structure, comprising: providing asemiconductor substrate, having a first pad and a second pad on a firsttop surface of the semiconductor substrate; providing a circuit boardhaving a conductive pad and an insulative pad, wherein a second topsurface of the conductive pad is at substantially same level as a thirdtop surface of the insulative pad; providing a first solder ball and asecond solder ball on the conductive pad and the insulative padrespectively; attaching the first pad and the second pad on the firstsolder ball and the second solder ball respectively; and reflowing thefirst solder ball and the second solder ball to form a first bump and asecond bump respectively and to increase an aspect ratio of the firstbump, wherein the first bump is arranged to have a first contact angleformed between a first contact surface of the first bump on theconductive pad and a surface of the conductive pad, the second bump isarranged to have a second contact angle formed between a second contactsurface of the second bump on the insulative pad and a surface of theinsulative pad, and the first contact angle is different from the secondcontact angle.
 10. The method of claim 9, wherein the first contactangle and the second contact angle are greater than 90 degrees, and thefirst contact angle is smaller than the second contact angle.
 11. Themethod of claim 9, further comprising: providing a first recessedportion recessed into the surface of the circuit board for accommodatingthe conductive pad; providing a second recessed portion recessed intothe surface of the circuit board for accommodating the insulative pad.12. The method of claim 9, wherein a maximum width of the first bump isdifferent from a maximum width of the second bump.
 13. The semiconductorstructure of claim 12, wherein the maximum width of the second bump isgreater than the maximum width of the first bump.
 14. A method forforming a semiconductor structure, comprising: providing a semiconductorsubstrate, having a plurality of first pads and a plurality of secondpads on a first top surface of the semiconductor substrate; providing acircuit board having a plurality of conductive pads and a plurality ofinsulative pads, wherein the plurality of conductive pads include aplurality of second top surfaces respectively, the plurality ofinsulative pads include a plurality of third top surfaces respectively,the plurality of third top surfaces are at substantially higher levelthan the plurality of second top surfaces; providing a plurality offirst solder balls respectively on the plurality of conductive pads, anda plurality of second solder balls respectively on the plurality ofinsulative pads; attaching the plurality of first pads and the pluralityof second pads on the plurality of first solder balls and the pluralityof second solder balls respectively; and reflowing the plurality offirst solder balls and the plurality of second solder balls to form aplurality of first bumps and a plurality of second bumps respectivelyand to increase a gap between the semiconductor substrate and thecircuit board, wherein the plurality of conductive pads and theplurality of insulative pads are formed as an array on the circuitboard, and the plurality of insulative pads are arranged to besymmetrical to a center of the array.
 15. The method of claim 14,further comprising: forming a plurality of recessed portions recessedinto a surface of the circuit board for accommodate the plurality ofconductive pads respectively.
 16. The method of claim 14, wherein theplurality of insulative pads are formed inside the array.
 17. The methodof claim 14, wherein the plurality of insulative pads are formed on aplurality of corners of the array respectively.
 18. The method of claim14, wherein the plurality of insulative pads are a plurality ofpolymeric pads respectively.
 19. The method of claim 14, whereinreflowing the plurality of first solder balls and the plurality ofsecond solder balls to form the plurality of first bumps and theplurality of second bumps respectively comprises: reflowing theplurality of first solder balls and the plurality of second solder ballsto form the plurality of first bumps wetted on the plurality ofconductive pads and the plurality of second bumps not wetted on theplurality of insulative pads respectively.
 20. The method of claim 14,wherein each of the plurality of first solder ball has a first amount ofsolder material, each of the plurality of second solder ball has asecond amount of solder material, and the first amount of soldermaterial is equal to the second amount of solder material.